Crate restructuring (#590)

* Nuke V2

* fmt

* Formatting and imports

* Updated benches

* Fix doc comment link

* Brace imports in ws-server

* Reworking imports

* std first

* fmt

* std on top

* Update to match changed line numbers

* Rename jsonrpsee_utils -> jsonrpsee_core

* Migrating things types -> core

* RpcError -> ErrorResponse

* Merge types::client into core::client

* Continued move types -> core

* Removing features to make checks pass

* Move rpc_module tests to tests crate

* Fixed doc comment links

* Add futures-util dependency for client

* Remove dead code

Co-authored-by: Niklas Adolfsson <[email protected]>

* fmt

* Feature gate the Client trait

* Move `Client` traits to `client` module

Co-authored-by: Niklas Adolfsson <[email protected]>